Question

determine a soma dos dezoitos primeiros termos da P.A (1,4,7...)

Answer 1

a1=1
r = 4-1 = 3
n= 18

an=a1+(n-1).r

a18 = 1+(18-1).3 ==> 1+17.3==> 1+51==> 52
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Sn= (a1+an).n
             2

S18= (1+52).18
               2

S18 = 53.9 ==> s18 = 477